Co-activation of antibody-responsive, enzymatic sensors by a recombinant scFv antibody fragment produced in E. coli

The biophysical nature of the signal transduction in enzymatic biosensors through which the paratope-epitope interaction enhances enzyme activity, is essentially unknown. Fab fragments of efficiently activating antibodies are, in general, poor sensor activators suggesting that the bivalent antibody binding could contribute to the sensing process. We have cloned and produced in E. coli a recombinant SD6 scFv fragment directed against a sensing peptide, displayed on a model -galactosidase-based biosensor. While the enzymatic response to scFv-binding is not detectable, the simultaneous presence of scFv and the poorly-activator SD6 Fab fragment results in a non-additive, efficient sensor response, enhancing the enzyme activity up to about 200%. This co-operative effect, which is also observed by combining SD6 scFv and the non-homologous anti-peptide 4C4 Fab, confirms that the enzyme up-regulation requires multiple and probably heterogeneous contacts between the sensor molecule and the analytes, but not necessarily done by bivalent molecular ligands.